2-FLUID INTERPRETATION OF THE MICROWAVE CONDUCTIVITY OF YBA2CU3O7-DELTA

The use of the two-fluid model for analyzing the microwave conductivit y of YBa2Cu3O7-delta is critically appraised for s-wave and d-wave mod els and for weak and strong scattering, in the light of published resu lts on single crystals, and of results here presented on high-quality epitaxial films and powders doped with Zn and Co. It is argued that th e normal electrons in the best samples show nonlocal conductivity (lam bda < l below 40 K), and that this provides a natural explanation of t he low-temperature behavior of the measured surface impedance. If corr ect this model is firmly in favor of d-wave pairing. Evidence is shown that some 10% of electrons remain normal at T = 0 in good films and 5 0% or more in heavily doped powders, in states which may or may not be localized. This behavior is associated with a T-2 term in the tempera ture dependence of lambda and is in accord with recent calculations of the effects of strong scattering impurities. It is argued that in the superconducting state transport between localized states is not neces sarily associated with hopping conduction, because of the effects of A ndreev reflection.